Adhesion promoting compounds are used in commercial ink formulations for packing inks in order to enhance the adhesion between the ink and the substrate onto which the ink is printed. Failure of adhesion leads to difficulties in the printing process or adhesion failures resulting in removal of the printed ink. Printing of food packaging on polymeric films is widespread and it is particularly important in such applications that the printed ink remains on the packaging and that parts of the ink composition do not contaminate the product packed in the packaging. Polymer films such as polypropylene (PP) and polyester (PE) need to be surface treated to provide the required adhesion of the ink to the film. The surface energy of a packaging film, expressed in the units dynes/cm, is a critical film property when printing, coating or laminating packaging films.
For PP films, the surface energy must be between 36-42 dynes/cm to achieve good adhesion, and for PE films, the surface energy must be above 52 dynes/cm to achieve good adhesion. If the surface energy goes down these range/values, then the adhesion promoters of the known art do not provide good adhesion.
Furthermore, the PP surface treated films tend to loose surface energy (surface charge) particularly when the film is surface treated by Corona to 38 dynes/cm, which further reduces to less than 36 dynes/cm. Therefore, use of PP films (and similarly of PE films) has substantial economic disadvantage, because the film has to be retreated again.
Adhesion promoters based upon titanium compounds have been known and used commercially for many years in solvent-borne packaging inks.
The common adhesion promoters used are
i) titanium acetylacetonates; and
ii) titanium alkyl phosphates.
i) The titanium acetylacetonates provide good adhesion between the ink and a treated film substrate. However, they tend to impart a) undesirable yellow color and b) undesirable odor to the ink. Furthermore, the titanium acetylacetonates c) are not recommended for use in the inks for food packaging due to toxicity issues of acetyl acetone.
ii) The U.S. Pat. Nos. 4,659,848 and 4,705,568 describe titanium phosphate adhesion promoters, which consists of a reaction product of a titanium orthoester and an alkyl phosphate. Such promoters provide efficient adhesion promotion with much less yellowing and odor than the aforementioned titanium acetylacetonates and can be used on food packaging. However, even these promoters do not provide good adhesion on low surface treated films. The adhesion promoter, Tyzor® IAM, is available from as available from Dorf Ketal Speciality Catalysts, or Dorf Ketal Chemicals (India) Private Limited in the comparative examples are in this family of adhesion promoters. The adhesion promoters described in Example 1 and 2 are in this family of adhesion promoters.
Recently there has been a degradation of the quality of the film substrates used in flexible packaging, which results in inconsistent surface treatments across the films, particularly when the film is treated by corona discharge surfaced treatment to create reactive groups for the adhesion promoters. For example, the PP films can have areas of the film with the surface energy equal to or less than about 36 dynes/cm, and the PE films may have areas with the surface energy equal to or less than about 52 dynes/cm, which has been found to be responsible for causing poor adhesion of the ink to the polymer films. This in turn creates a performance gap which must be overcome.